1. Field of the Invention
The present invention relates to a rotary pump.
2. Description of Related Art
An internal gear pump, which pumps fluid (e.g., oil), is known. In general, the internal gear pump includes an inner rotor, which has external teeth along an outer peripheral part thereof, and an outer rotor, which has internal teeth along an inner peripheral part thereof. The inner rotor and the outer rotor are arranged eccentric to each other while the external teeth of the inner rotor and the internal teeth of the outer rotor are meshed with each other. When the inner rotor and the outer rotor are rotated, a volume of a pressure chamber, which is formed between the external teeth and the internal teeth, changes, so that the fluid is drawn and discharged at the gear pump.
When a volumetric efficiency of the internal gear pump, which is a ratio between an actual discharge rate and a theoretical discharge rate (or a ratio between an actual flow rate and a theoretical flow rate) of the internal gear pump, needs to be increased, it is required to minimize each corresponding clearance, such as a clearance between the inner rotor and a housing, and a clearance between the outer rotor and the housing. For instance, in Japanese Unexamined Patent Publication No. 2004-11520A (US200310227216A1), a side plate is placed on a side of the outer rotor, and the discharge pressure of the fluid is applied to a back surface of the side plate to reduce the size of the clearance and thereby to improve the sealing performance. In this way, the volumetric efficiency is improved.
When the clearance between the inner rotor and the housing or between the outer rotor and the housing is decreased, like in the case of Japanese Unexamined Patent Publication No. 2004-11520A (US2003/0227216A1), a drive torque, which is required to drive the pump, is disadvantageously increased. Furthermore, in the case of Japanese Unexamined Patent Publication No. 2004-11520A (US2003/0227216A1), the additional components, such as the side plate, are required, so that the structure of the pump is disadvantageously complicated, and the number of the components of the pump is disadvantageously increased.